This invention relates to the field of telephone signal processing. More particularly, this invention relates to method and apparatus for intercoupling a key telephone instrument with an electronic private branch exchange.
Telephone instruments are known which are provided with the capability of handling incoming or outgoing calls on a plurality of voice frequency lines. Such telephone instruments, generically termed key telephone instruments (e.g. a key telephone handset or a desk top unit marketed under the Trade Mark CALL DIRECTOR.sup.TM are typically provided with a line selector unit comprising a plurality of manually operable line push button switches which enable the operator to select one of the plurality of lines for active operation. The line selector unit also includes a hold push button switch which the operator may depress to place a call on hold. Such telephone instruments are also typically provided with line status indicators, such as a plurality of incandescent lamps which indicate by illumination which of the plurality of lines is active, a ringing circuit including a bell for indicating an incoming call from an outside line (ring), and optionally a buzzer for indicating an incoming call on an intercom line (buzz).
In the past, in order to enable proper functional operation of a key telephone instrument, three pairs of conductors (i.e. six conductors) have been required for each line in a key telephone or call director. For a key telephone having a six line capability, for example, 36 individual conductors have been required, each of which must be connected to the master bus carrying the individual subscriber lines in the branch exchange. Due to the relatively large number of conductors required, it is economically undesirable to provide more than a few key telephone instrument outlets in an office when the private branch exchange wiring system is being installed. Thus, the sites for key telephone handsets and call directors must be preselected in advance and cannot be changed without extensive rewiring or replacement of the master bus, both of which are undesirable limitations.
Due to recent developments in the field of electronic telephone switching systems, it is now possible to provide a fully electronic private branch exchange in which each key telephone instrument is tied into the system by means of a conventional voice conductor pair over which the audio frequency voice information is conducted and a serial data conductor pair over which supervisorial serial data signals are conducted.